Numerous methods have already been proposed for measuring the velocity of a fluid in steady and continuous flow.
By way of example, mention may be made of direct methods that make use of floats, timed photography, a laser velocity meter, or sinners, and indirect methods that make use of the measured dynamic pressure in the fluid, the measured density of the flowing fluid, or methods that make use of a hot wire or film.
For non-viscous incompressible fluids flowing in steady and continuous flow, the velocity measurement method based on measuring the kinetic pressure makes use of Bernoullo's equation in its integrated form: EQU p+.rho.gz+1/2.rho.U.sup.2 =constant (1)
where:
p=the local static pressure in the fluid; PA1 .rho.=the local density of the fluid; PA1 g=the acceleration due to gravity; PA1 z=altitude; and PA1 U=the modulus of the velocity. PA1 i) detecting instants at which the acceleration of the fluid is zero; PA1 ii ) defining a measurement of the fluid velocity at instants where the acceleration is zero; and PA1 iii) determining changes in velocity between instants of zero acceleration by numerically integrating the difference between two static pressures.
for gases, variations in .SIGMA.gz may be considered as being zero.
The impact pressure or total pressure p.sub.i may be expressed in the form: EQU p.sub.i =p+1/2.rho.U.sup.2 ( 2)
The modulus of the velocity can thus be obtained by taking the difference between the impact pressure p.sub.i and the static pressure p: EQU U=[2(p.sub.i -p)/.rho.].sup.1/2 ( 3)
However, unsteady flows make velocity measurement operations particularly difficult. Various velocity measurements used for fluids in steady and continuous flow are not applicable to fluids in unsteady flow.
In particular, a velocity measurement based on measuring kinetic pressure as outlined above is not usable with unsteady flows. Bernoulli's equation (1) recalled above is not applicable thereto since, as indicated below, the fluid mechanics equations for unsteady flow include, in particular, the time derivative of velocity.
Consequently, the methods commonly applied in the past for measuring the velocity of fluids in unsteady flow consists essentially in using a hot wire anemometer or a Doppler effect laser velocity meter. However, it is not easy to implement these techniques that are expensive, difficult, and sophisticated, and they are particularly ill-suited to use on an industrial site.
An object of the present invention is to eliminate the drawbacks of the prior art.